For hydrostatic propulsion drives which are to provide exact transmission ratios and are to propel in an exactly speed-controlled manner, an electro-proportional (EP) pivot angle regulation is typically used both for the hydraulic pump and for the hydraulic motor. Typical applications are harvesting machines, combine harvesters, lift trucks, tractors with a power-split transmission, field sprayers, municipal vehicles and forestry machines. In part load operation, in which the operating pressure is low, the degree of efficiency of hydrostatic units (hydraulic pump or hydraulic motor) is typically poorer than under full load, since volumetric flow-induced pressure drops via lines and reduced cross sections in cast parts have a greater percentage effect at a low operating pressure, and since a low operating pressure leads to lower leakage flows via gaps between moving parts than a high operating pressure, and the friction increases as a result.
In EP 1 754 643 B1, the operating point of a diesel engine and a hydrostatic transmission is optimized during the running operation with regard to a low fuel consumption. In practice, this leads to the diesel engine rotational speed and the swept volumes of the hydrostatic machines frequently changing dynamically in a manner which is dependent on the load and the driver's request. This can be perceived as unpleasant by the driver.